In the following description, explained is a method of transmitting packets in terminals that use IP based voice over internet protocol (hereinafter abbreviated ‘VoIP’) in a broadband wireless access system. For this, VoIP traffic is described as follows. Yet, a packet transmitting method according to the present invention needs not to be limited to the VoIP packet transmission described in the following.
First of all, VoIP traffic is characterized in being generated in a fixed size with a fixed period in VoIP codec. And, VoIP communication can be divided into a talk period (talk-spurt), for which inter-user call is in progress, and a silence period for which a user is not talking but listening. In general, the silence period occupies over 50% in a whole call session.
In order to allocate bandwidths differing in size to the talk-spurt and the silence period, various kinds of audio codecs are used. A representative one of the various kinds of audio codecs is AMR (adaptive multi-rate) scheme used by GSM (global system for mobile communication) or UMTS (universal mobile telecommunication system).
Since voice data is not generated for the silence period, if a bandwidth is allocated to the silence period, it may bring about a resource waste. To prevent the resource waste, VoIP supports a silence suppression scheme. According to the silence suppression scheme, a vocoder for generating VoIP traffic does not generate traffic for the silence period and periodically generates a comfort noise to inform a correspondent user that a call keeps being maintained. For instance, a vocoder, which uses the. AMR codec, generates a packet in a fixed size once per 20 ms for a talk-spurt or generates a comfort noise per 160 ms.
Meanwhile, for resource allocation to the traffic having a predetermined period in a fixed size like VoIP in general, a base station is able to use a method of allocating a designated region to a specific terminal fixedly. Namely, a region having an initially determined size is allocated to a terminal supporting the VoIP service. And, it is able to inform the terminal of the allocated region information via an initially transmitted control channel or message (e.g., UL-MAP or DL-MAP). Thus, the initially transmitted control channel or message can contain period information of a region that will be allocated in the future as well.
Subsequently, for a next period, the base station is able to keep allocating the corresponding region having been notified to the terminal via the initially transmitted control channel or message without a special notice. Therefore, the terminal transmits VoIP packets via the allocated region using the region information allocated by a MAP in the early stage and then transmits VoIP packets via the same region from a next period using period information.
Assume that a length of a frame is set to 5 ms to consider a VoIP service. And, assume that a frame period allocated to a terminal for VoIP packet transmission is set to 4 frames. In this case, a frame period allocated to a terminal for VoIP packet transmission may vary according to characteristics of a service. In particular, in case of a same VoIP service, the frame period allocated for the VoIP packet transmission may be used by being defined different according to the respective consideration factors such as system characteristics (e.g., a system characteristic according to a frame length), a VoIP service status (e.g., a talk-spurt, a silence period) and the like.
In an initial frame, a base station notifies a terminal of allocated region information for VoIP packet transmission via UL-MAP. Thereafter, in a fourth frame or an eighth frame corresponding to each period, the base station does not announce region information via the UP-MAP but allocates a region for the VoIP packet transmission only.
In this case, a period allocated for the VoIP packet transmission corresponds to four frames (i.e., 20 ms). In particular, the terminal keeps storing the region allocation information contained in the UL-MAP received in the initial frame and is then able to transmit a VoIP packet via a corresponding region if there is not reception of UL-MAP in the fourth or eighth frame. Thus, the base station is able to allocate the resource for a single VoIP connection fixedly and constantly due to the VoIP traffic characteristics.
In the following description, briefly described is a method of transmitting data between a transmitting side and a receiving side. In a data transmitting method, if a transmission failure occurs, a receiving side makes a retransmission request for corresponding data. In this case, ARQ (automatic repeat request) scheme is generally used as a data retransmission scheme.
In the ARQ scheme, an acknowledgement/non-acknowledgement (hereinafter abbreviated ACK/NACK) signal indicating whether a receiving side correctly receives data is notified to a transmitting side. The receiving side then retransmits the data for the corresponding signal in case of receiving the NACK signal. The ARQ scheme can be categorized into SAW (stop-and-wait) ARQ, GBN (go-back-N) ARQ and SR (selective-repeat) ARQ.
In the SAQ ARQ scheme, a transmitting side waits after data transmission until receiving ACK or NACK signal. If the ACK signal is received, the transmitting side newly transmits next data. If the NACK signal is received, the transmitting side retransmits the previous data. Namely, the SAW ARQ scheme is a scheme for transmitting a single frame at a time only. If it is confirmed that a corresponding frame is successfully delivered, a next frame is transmitted.
The GBN ARQ scheme is a scheme for continuously transmitting data regardless of a response message. In case that a receiving side fails to receive data of a specific frame in the course of receiving data, the receiving side is unable to transmit an ACK signal for the specific frame to a transmitting side. Since the transmitting side is unable to receive the ACK signal for the specific frame, the transmitting side retransmits data from the data of the specific frame.
In the SR ARQ scheme, while data keeps being transmitted, data corresponding to a received NACK signal is retransmitted only. If a receiving side fails to receive data of a specific frame, the receiving side transmits a NACK signal to a transmitting side. The transmitting side having received the NACK signal then retransmits the data of the frame indicated by the NACK signal to the receiving side to transmit the whole data. Since the SR ARQ scheme gives a sequence to each frame and manages it, the implementation of the SR ARQ scheme may becomes complicated relatively.
In the scheme for transmitting data in packet format, as a higher data rate becomes necessary, a coding rate or a modulation scheme, which has a suitable level to prevent error generated in high-speed transmission environment, is applied to a communication system. And, ARQ scheme suitable for the high-speed transmission environment, i.e., Hybrid ARQ (hereinafter abbreviated HARQ) has been proposed.
In the ARQ scheme, if error is generated, the corresponding information is discarded. Yet, in the HARQ scheme, a receiving side stores erroneous information in s buffer. The receiving side combines the stored information with retransmitted information and then applies FEC (forward error correction) thereto. Namely, the HARQ scheme may be regarded as the scheme generated from combining the ARQ scheme with the FEC. The HARQ can be mainly categorized into the following four types.
In a first type HARQ scheme, a receiving side always checks an error detection code in data and then primarily applies FEC thereto. If error still remains in a packet, the receiving side makes a request for a retransmission to a transmitting side. The receiving side discards an erroneous packet. The transmitting side then retransmits a packet by applying the same FEC code of the discarded packet to the retransmitted packet.
Second type HARQ scheme is called IR (incremental redundancy) ARQ scheme. In the second type HARQ scheme, a receiving side stores a first transmitted packet in a buffer instead of discarding it and then combines it with retransmitted redundancy bits. In case of retransmission, a transmitting side retransmits parity bits only except data bits. Each time the parity bits are retransmitted by the transmitting side, different ones are used.
A third type HARQ scheme corresponds to a special case of the second type HARQ scheme. Each packet is self-decodable. In case that a transmitting side performs retransmission, the transmitting side configures a packet containing an erroneous part and data together and then retransmits the corresponding packet. Although this scheme enables decoding more accurate than the second type HARQ scheme, it has efficiency poorer than that of the second type HARQ scheme in aspect of coding gain.
In a fourth type HARQ scheme, a function of combining the data initially received and stored by a receiving side with retransmitted data is added to a function of the first type HARQ scheme. And, the fourth type HARQ scheme may be called a matrix combining scheme or a chase combining scheme. Moreover, the fourth type HARQ scheme has a gain in aspect of SINR (signal to interference noise ratio) and always uses the same parity bits of the retransmitted data.
In case that error is generated in data transmission or data is lost in the data transmission, the above-described data transmitting methods enable reconstruction of original data.